1. Field of the Invention
The present invention relates to a drive circuit for driving an ultrasonic motor and a method of driving and controlling the ultrasonic motor.
2. Related Art
Conventionally, ultrasonic motors have been known in which ultrasonic vibration is utilized to generate a driving force. In a traveling-wave type ultrasonic motor, a piezoelectric body is adhered to an annular elastic body to form a stator, against which a rotor attached to a drive shaft is pressed so as to contact the stator. A drive circuit for the ultrasonic motor supplies the piezoelectric body with a sine-wave drive signal and a cosine-wave drive signal of predetermined frequencies. Due to mechanical vibration generated by these two drive signals, an ultrasonic vibration (traveling wave) whose loops and nodes travel in a circumferential direction along the elastic body is excited in the elastic body. Due to the traveling wave, the rotor which contacts the elastic body and the drive shaft are rotated.
The amplitude of the mechanical vibration generated in the piezoelectric body becomes a maximum when the frequencies of the drive signals coincide with the resonant frequency. However, within a specific frequency band including the resonant frequency, abnormal vibrations of an audible frequency are generated in the elastic body, and the vibrations lower the rotational speed of the rotor and the efficiency of the ultrasonic motor. Accordingly, the ultrasonic motor is driven such that drive signals whose frequencies are sufficiently higher than a frequency band in which audible sound is generated (hereinafter referred to as "audible sound generating band") are initially supplied to the ultrasonic motor. The frequencies of the drive signals are then gradually lowered so as to enter a drive frequency band slightly higher than the audible sound generating band and are then maintained within this drive frequency band.
The audible sound generating band and the drive frequency band of the ultrasonic motor change depending on the ambient temperature of the ultrasonic motor and the magnitude of the load acting on the ultrasonic motor. Accordingly, the proper frequencies of the drive signals must not be constant and must be changed in accordance with the ambient temperature, the load and the like. Therefore, a device has been proposed in which a piezoelectric element is adhered to the elastic body so as to control the frequencies of the drive signals based on an AC detection signal which is output from the piezoelectric element in accordance with the ultrasonic vibration of the elastic body.
As an example of frequency control of drive signals, a technique is disclosed in Japanese Patent Application Laid-open No. 62-203575 in which a detection signal output from a piezoelectric element is subjected to half-wave rectification by a diode, followed by smoothing with a capacitor to obtain a smoothed signal. The frequency is controlled such that the level of the smoothed signal becomes a predetermined value lower than a predetermined level which is obtained at the above-mentioned resonant frequency. Another technique is disclosed in Japanese Patent Application Laid-open No. 3-159583 in which irregularity of the waveform of a detection signal output from a piezoelectric element is monitored, and the frequencies of drive signals are lowered when irregularity of the waveform does not occur. On the contrary, when irregularity of the waveform occurs, it is judged that the frequencies of the drive signals have entered the audible sound generating band, and the frequencies are raised.
In the frequency control arrangement disclosed in Japanese Patent Application Laid-open No. 62-203575, the level of a signal obtained by smoothing the detection signal output from the piezoelectric element, namely, the average level of the detection signal, is compared with a predetermined level. Therefore, even when the frequencies of the drive signals enter the audible sound generating band and irregularity of the waveform occurs, the frequencies are lowered if the average level of the detection signal is lower than the above-mentioned predetermined level. Accordingly, this frequency control arrangement has the drawback that the frequencies of the drive signals enter the audible sound generating band so that audible sound is generated from the ultrasonic motor.
Further, among various ultrasonic motors, there are some ultrasonic motors in which irregularity of the waveform of the detection signal does not occur even when the frequencies of the drive signals enter the audible sound generating band because of their shapes and sizes, pressing force of the rotor and the stator, and other factors. When the frequency control method disclosed in Japanese Patent Application Laid-open No. 3-159583 is used for driving the above-mentioned ultrasonic motor, the frequencies of the drive signals are lowered and enter the audible sound generating band, and are further lowered, passing the resonant frequency, because irregularity of the waveform of the detection signal does not occur even when the frequencies of the drive signals enters the audible sound generating band. Accordingly, this frequency control method is not suitable for ultrasonic motors in which irregularity of the waveform of the detection signal does not occur in the audible sound generating band.